National Weather Service United States Department of Commerce

Weather Glossary: M's

One of 2 categories of downbursts (the other category is called a microburst). This Downburst has an affected outflow area of at least 2.5 miles (4 kilometers) wide and peak winds lasting between 5 and 20 minutes. Intense macrobursts may cause tornado-force damage up to F-3.
Large scale, characteristic of weather systems several hundred to several thousand kilometers in diameter.
Main Lobe
The envelope of electromagnetic energy along the main axis of the radar beam.
Main Stem
The reach of a river/stream formed by the tributaries that flow into it.
Major Flooding
A general term including extensive inundation and property damage. (Usually characterized by the evacuation of people and livestock and the closure of both primary and secondary roads.)
Mammatus Clouds
Rounded, smooth, sack-like protrusions hanging from the underside of a cloud (usually a thunderstorm anvil). Mammatus clouds often accompany severe thunderstorms, but do not produce severe weather; they may accompany non-severe storms as well.
MAP (Mean Areal Precipitation)
The average rainfall over a given area, generally expressed as an average depth over the area.
Marine Inversion
A temperature inversion created by the cooling of a warm airmass from below by the cool lakes on spring and summer days. The same effect can occur along the oceans anytime of the year.
Marine Small Craft Thunderstorm Advisory
A marine warning issued by Environment Canada Atmospheric Environment Branch when the possibility of thunderstorms is greater than 40% chance.
Marine Small Craft Wind Warning
A marine warning issued by Environment Canada Atmospheric Environment Branch for winds which are forecasted to be in the 20-33 knot range inclusive.
Marine Weather Statement
The National Weather Service will issue this statement:
  1. To provide follow-up information on Special Marine Warnings and to cancel all or part of a warning.
  2. To describe short duration, non-severe, but potentially hazardous conditions which sustained winds or frequent gusts are less than 34 knots for 2 hours or less. Short-lived increases in winds, although below threshold for Special Marine Warnings, that may make small craft handling difficult especially for inexperienced boaters.
  3. To provide information for a variety of conditions not covered by warnings or routine forecasts (e.g., low water conditions, dense fog, etc.).
  4. To discuss increasing or decreasing winds and to convey details on possible later warnings.
Mass Curve
A graph of the cumulative values of a hydrologic quantity (such as precipitation or runoff), generally as ordinate, plotted against time or date.
An abbreviation of maximum.
Max Parcel Level (MPL)
This signifies the highest attainable level that a convective updraft can reach; therefore, it is a good indication of how tall a thunderstorm may reach.
Maximum Spillway Discharge
Spillway discharge (cfs) when reservoir is at maximum designed water surface elevation.
Maximum Unambiguous Range
The greatest distance a pulse can travel and return before the next pulse is transmitted. R_max = c / (2*PRF), where c is the speed of light, PRF is pulse repetition frequency.
Maximum Unambiguous Velocity
The maximum radial velocity that can be detected without velocity aliasing. V_max = PRF * L / 4, where PRF is pulse repetition frequency, L is wavelength.
The winding of a stream channel.
Meander Belt
The area between lines drawn tangential to the extreme limits of fully developed meanders.
Mean Areal Precipitation
The average rainfall over a given area, generally expressed as an average depth over the area.
Mean Depth
The average depth of water in a stream channel or conduit. It is equal to the cross-sectional area divided by the surface width.
Mean Doppler Velocity
Reflectivity-weighted average velocity of targets in a given pulse resolution volume. Usually determined from a large number of successive radar pulses. Also called mean radial (towards or away from the antenna) velocity. Doppler velocity refers to spectral density first moment, radial velocity to base data. It is also called mean radial velocity.
Mean Low Water (MLW)
The average height of the daily low tides recorded over a 19-year period at a specific location.
Mean Lower Low Water (MLLW)
The average height of the lower of the two low tides occurring during a tidal cycle recorded over a 19-year period at a particular location.
Mean Sea Level (MSL)
The average height of the surface of the sea at a particular location for all stages of the tide over a 19-year period. This is usually determined from the hourly height readings of the tide gage at that site.
Mean Temperature
The average of a series of temperatures taken over a period of time, such as a day or a month.
Precipitation of 0.01" (0.25 mm) or more.
Medium Range
In forecasting, (generally) three to seven days in advance.
Melting Level
The altitude which ice crystals and snowflakes melt as they descend through the atmosphere.
The curved surface of the liquid at the open end of a capillary column.
Meridional Flow
Large-scale atmospheric flow in which the north-south component (i.e., longitudinal, or along a meridian) is pronounced. The accompanying zonal (east-west) component often is weaker than normal. Compare with zonal flow.
Mesocyclone (M)
This WSR-88D radar product displays information regarding the detection of the following 3 types of azimuthal shear patterns:
  1. Uncorrelated Shear - It is a sufficiently strong circulation detected on only one elevation angle within a thunderstorm.
  2. 3-Dimensional Correlated Shear - It is a sufficiently strong circulation detected on two or more elevation angles within a thunderstorm, but less than two of the features are symmetrical. It is displayed on the radar display as a thin yellow circle.
  3. Mesocyclone - It is a sufficiently strong circulation detected on two or more elevation angles which are symmetrically linked within a thunderstorm.
Mesocyclone (MESO)

A storm-scale region of rotation, typically around 2-6 miles (3-10 km) in diameter and often found in the right rear flank of a supercell (or often on the eastern, or front, flank of an HP storm).

The circulation of a mesocyclone covers an area much larger than the tornado that may develop within it. Properly used, mesocyclone is a radar term; it is defined as a rotation signature appearing on Doppler radar that meets specific criteria for magnitude, vertical depth, and duration.

It will appear as a yellow solid circle on the Doppler velocity products. Therefore, a mesocyclone should not be considered a visually-observable phenomenon (although visual evidence of rotation, such as curved inflow bands, may imply the presence of a mesocyclone).

A relatively small area of high atmospheric pressure that forms beneath a thunderstorm. It is usually associated with MCSs or their remnants.
Mesolow (or Sub-synoptic Low)
A mesoscale low-pressure center. Severe weather potential often increases in the area near and just ahead of a mesolow. Mesolow should not be confused with mesocyclone, which is a storm-scale phenomenon.
A regional network of observing stations (usually surface stations) designed to diagnose mesoscale weather features and their associated processes.
Size scale referring to weather systems smaller than synoptic-scale systems but larger than storm-scale systems. Horizontal dimensions generally range from around 50 miles (80 kilometers) to several hundred miles. Squall lines, MCCs, and MCSs are examples of mesoscale weather systems.
Mesoscale Convective Complex (MCC)

A large MCS, generally round or oval-shaped, which normally reaches peak intensity at night. The formal definition includes specific minimum criteria for size, duration, and eccentricity (i.e., "roundness"), based on the cloud shield as seen on infrared satellite photographs:

  • Size - Area of cloud top -32°C or less: 100,000 square kilometers or more (slightly smaller than the state of Ohio), and area of cloud top -52°C or less: 50,000 square kilometers or more.
  • Duration - Size criteria must be met for at least 6 hours.
  • Eccentricity - Minor/major axis at least 0.7.

MCCs typically form during the afternoon and evening in the form of several isolated thunderstorms, during which time the potential for severe weather is greatest. During peak intensity, the primary threat shifts toward heavy rain and flooding.

Mesoscale Discussions (MCD)

When conditions actually begin to shape up for severe weather, SPC often issues a Mesoscale Discussion (MCD) statement anywhere from roughly half an hour to several hours before issuing a weather watch.

SPC also puts out MCDs for hazardous winter weather events on the mesoscale, such as locally heavy snow, blizzards and freezing rain (see below). MCDs are also issued on occasion for heavy rainfall, convective trends, and other phenomena, when the forecaster feels he/she can provide useful information that is not readily available or apparent to field forecasters.

MCDs are based on mesoscale analysis and interpretation of observations and of short term, high resolution numerical model output.

The MCD basically describes what is currently happening, what is expected in the next few hours, the meteorological reasoning for the forecast, and when/where SPC plans to issue the watch (if dealing with severe thunderstorm potential).

Severe thunderstorm MCDs can help you get a little extra lead time on the weather and allow you to begin gearing up operations before a watch is issued.

Mesoscale Convective System (MCS)

A complex of thunderstorms which becomes organized on a scale larger than the individual thunderstorms, and normally persists for several hours or more.

MCSs may be round or linear in shape, and include systems such as tropical cyclones, squall lines, and MCCs (among others). MCS often is used to describe a cluster of thunderstorms that does not satisfy the size, shape, or duration criteria of an MCC.

A weather observation near ground level. It may include date and time, wind, visibility, weather and obstructions to vision, sky condition, temperature and dew point, sea level pressure, precipitation amount and other data used for aircraft operations.
Meteoric Water
Water derived from precipitation.
The study of the atmosphere and atmospheric phenomena.

One of 2 categories of downbursts (the other category is called a macroburst). This downburst has an affected outflow area of less than 2.5 miles (4 kilometers) wide and peak winds lasting less than 5 minutes. They may induce dangerous horizontal/vertical wind shears, which can adversely affect aircraft performance and cause property damage.

They can be sub-classified into either dry or wet microburst depending on how much (or little) rain accompanies the microburst when it reaches the ground. Most microbursts are rather short-lived (5 minutes or so), but on rare occasions they have been known to last up to 6 times that long.

Middle Clouds
In the middle family are the altostratus, altocumulus, and nimbostratus clouds. The height of the bases of these clouds ranges from 6,500 to 23,000 feet (2,000 to 7,00 meters) in middle latitudes. These clouds are primarily water; however, much of the water may be supercooled and the clouds can contain some ice crystals.
Mid-Latitude Areas
Areas between 30° and 60° north and south of the Equator.
Mid-level Cooling
Local cooling of the air in middle levels of the atmosphere (roughly 8,000 to 25,000 feet/2,400 to 7,600 meters), which can lead to destabilization of the entire atmosphere if all other factors are equal. Mid-level cooling can occur, for example, with the approach of a mid-level cold pool.
Mie Scattering
Any scattering produced by spherical particles whose diameters are greater than 1/10 the wavelength of the scattered radiation. This type of scattering causes the clouds to appear white in the sky. Often, hail exhibits in this type of scattering.
Millibar (mb)
Unit of atmospheric pressure. It is equal to 0.03" of mercury. One thousand millibars equals 29.55" of mercury on a barometer.
Miners' Inch
A rate of discharge through an orifice one inch square under a specific head.
Minor Flooding
A general term indicating minimal or no property damage but possibly some public inconvenience.
Minor Tidal Overflow
Minor flooding caused by high tides that results in little if any damage.
Mist (BR)
A visible aggregate of minute water particles suspended in the atmosphere that reduces visibility to less than 7 statue miles (11 kilometers), but greater than or equal to 5/8 statue miles (1 kilometer).
The scale of meteorological phenomena that ranges in size from 130 feet (40 meters) to about a 2.5 miles (4 kilometers). It includes rotation within a thunderstorm.
CAPE calculated using a parcel consisting of Mean Layer values of temperature and moisture from the lowest 100 mb above ground level.
Lifted Index (LI) calculated using a parcel consisting of Mean Layer values of temperature and moisture from the lowest 100 mb above ground level.
Air movements (usually vertical) that make the properties of the air with a parcel homogeneous. It may result in a lapse rate approaching the moist or dry adiabatic rate.
A mathematical representation of a process, system, or object developed to understand its behavior or to make predictions. The representation always involves certain simplifications and assumptions.
Model Output Statistics (MOS)
A set of statistical equations that use model output to forecast the probability of precipitation, high and low temperature, cloud cover, and precipitation amount for many cities across the USA. The statistical equations were specifically tailored for each location, considering factors such as each location's climate.
Moderate Flooding
The inundation of secondary roads; transfer to higher elevation necessary to save property - some evacuation may be required.
Moderate Icing
The rate of ice accumulation on an aircraft is such that even short encounters becomes potentially hazardous and the use of de-icing/anti-icing equipment or a diversion is necessary.
Moderate Risk of Severe Thunderstorms
Severe thunderstorms are expected to affect between 5 and 10 percent of the area. A moderate risk indicates the possibility of a significant severe weather episode. See high risk, slight risk, and convective outlook.
Moderate Turbulence
  1. Turbulence that is similar to Light Turbulence but of greater intensity. Changes in altitude and/or attitude occur but the aircraft remains in positive control at all times. It usually causes variations in indicated airspeed, or
  2. Turbulence that is similar to Light Chop but of greater intensity. It causes rapid bumps or jolts without appreciable changes in aircraft altitude or attitude. Report as Moderate Chop.
Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects are dislodged. Food service and walking are difficult.
Moist Adiabats
They show how the air temperature would change inside a rising parcel of saturated air.
Moisture Advection
Transport of moisture by horizontal winds.
Moisture Convergence
A measure of the degree to which moist air is converging into a given area, considering the effect of converging winds and moisture advection. Areas of persistent moisture convergence are favored regions for thunderstorm development, if other factors (e.g., instability) are favorable.
Moisture Equivalent
The ratio of:
  1. the weight of water which the soil, after saturation, will retain against a centrifugal force 1,000 times the force of gravity, to
  2. the weight of the soil when dry.
The ratio is stated as a percentage.
Monthly Climatological Report
This climatological product is issued once a month by each National Weather Service office. It is a mix of tabular and narrative information. It is organized so that similar items are grouped together (i.e., temperature, precipitation, wind, heating/cooling degree information, etc.).
Monthly Climatic Data of the World (MCDW)
A National Climatic Data Center publication which contains monthly mean temperature, pressure, precipitation, vapor pressure, and sunshine for approximately 2,000 surface data collection stations worldwide and monthly mean upper air temperatures, dew point depressions, and wind velocities for approximately 800 upper air stations.
A wind which blow from opposite directions between winter and summer. Usually the wind blows from land to sea in winter and from sea to land in summer.
Monstatic Radar
A radar that uses a common antenna for both transmitting and receiving.
Morning Glory
An elongated cloud band, visually similar to a roll cloud, usually appearing in the morning hours, when the atmosphere is relatively stable. Morning glories result from perturbations related to gravitational waves in a stable boundary layer. They are similar to ripples on a water surface; several parallel morning glories often can be seen propagating in the same direction.
An acronym for Model Output Statistics.
Mostly Clear
When the predominant/average sky condition is covered 1/8 to 2/8 with opaque (not transparent) clouds. Sometime called Mostly Sunny if it is during the day.
Mostly Cloudy
When the predominant/average sky condition is covered by more than half, but not completely covered by opaque (not transparent) clouds. In other words, 5/8 to 7/8 of the sky is covered by opaque clouds. Same as considerable cloudiness.
Mostly Sunny
When the predominant/average sky condition is covered 1/8 to 2/8 with opaque (not transparent) clouds. Same as mostly clear.
Movable Bed
A stream bed made up of materials readily transportable by the stream flow.
Moveable Bed Streams
These are most common in the arid West, where steep slopes and lack of vegetation result in a lot of erosion. During a flood, a channel may be eroded more deeply, or it may become filled with sediment and move to a different location.
MRF - Medium-Range Forecast model
The MRF was one of the main models' forecasters use for the medium range time period beyond 48 hours into the future. It was replaced by the Global Forecast System (GFS).
CAPE calculated using a parcel from a pressure level that results in the Most Unstable CAPE possible.
Colloquially descriptive of warm and especially humid weather.
Lifted Index (LI) calculated using a parcel from the pressure level that results in the Most Unstable value (lowest value) of LI possible. See Lifted Index.
Multicell Thunderstorms

These thunderstorms are organized in clusters of at least 2-4 short-lived cells. Each cell generates a cold air outflow and these individual outflows combine to form a large gust front.

Convergence along the gust front causes new cells to develop every 5 to 15 minutes. The cells move roughly with the mean wind. However, the area (storm) motion usually deviates significantly from the mean wind due to discrete propagation (new cell development) along the gust front.

The multicellular nature of the storm is usually apparent on radar with multiple reflectivity cores and maximum tops.

Severe multicell thunderstorms occur in environments possessing moderate vertical wind shear and moderate to large positive buoyancy (CAPE).

The moderate vertical wind shear leads to the development of a non-symmetric surface convergence pattern associated with the thunderstorm outflow, with the strongest convergence taking place on the downwind from the storm's motion.

New cells developing along this enhanced convergence zone tend to move in the same direction as the older cells, which increases the time that the new cells will experience low-level convergence and tap into the warm, moist inflow air. Cell regeneration in the convergence zone along an outflow boundary is the primary physical mechanism for maintaining a multicell storm. Moderate to large buoyancy ensures that strong updrafts are possible.

Multicell thunderstorms usually develop in an environment where the vertical wind shear is characterized by a straight-line hodograph. The name describes well what it looks like on the hodograph. The Bulk Richardson number for a multicellular thunderstorm environment is typically above 50.

These thunderstorms can become severe producing hail greater than 1" (2.5 cm) hail, wind gusts greater than 58 mph (93 km/h), and possibly a tornado. Nearly all thunderstorms (including supercells) are multi-cellular, but the term often is used to describe a storm which does not fit the definition of a supercell.

Multiple-vortex (or Multi-vortex) Tornado
A tornado in which two or more condensation funnels or debris clouds are present at the same time, often rotating about a common center or about each other. Multiple-vortex tornadoes can be especially damaging. See suction vortex.
Multipurpose Reservoir
A reservoir constructed and equipped to provide storage and release of water for two or more purposes such as flood control, power development, navigation, irrigation, recreation, pollution abatement, domestic water supply, etc.
Municipal Use of Water
The various uses to which water is put to use developed urban areas, including domestic use, industrial use, street sprinkling, fire protection, etc. The term is an inclusive one, applied where the uses are varied.
Slang for a thunderstorm with a well-defined anvil rollover, and thus having a visual appearance resembling a mushroom.
The fear of darkness.